A mobile WiMAX network based on IEEE (Institute of Electrical and Electronics Engineers) 802.16e/m or WiMAX standard allows transmitting much information in a short time owing to a broadband frequency, and also enables to improve efficiency in channel use since channels are shared among all users. Accordingly, the mobile WiMAX network has attracted considerable attention as a next-generation portable Internet system, whereby the mobile WiMAX network is currently deployed in many countries including U.S.A., Korea, Singapore, and etc.
WiMAX Forum dated on April, 2008 proposed the first WiMAX network capable of supporting mobility over WiMAX based on WiMAX network profile 1.0. After that, IEEE 802.16 Maintenance Technology Group (TG) develops a revision of IEEE 802.16e-2005 to improve a system performance, and expects to complete the revision by the end of 2010.
The system requirements of the mobile WiMAX network based on IEEE 802.16m mandates that the system shall provide the traffic throughput higher than two times of the mobile WiMAX network based on IEEE 802.16e. Especially, IEEE 802.16m concentrates on performance improvement in PHY layer and MAC layer, and improvement of End-To-End performance, to thereby satisfy the user's requirements and simultaneously arouse the consumer's interest in the market. Accordingly, IEEE 802.16m emphasizes the importance of performance beyond the PHY layer and MAC layer. Furthermore, IEEE 802.16m considers the improvement in traffic throughput of upper layer (TCP/IP layer) as well as traffic throughput of PHY layer and MAC layer.
A broadband wireless communication network based on IEEE 802.16e/m converts data to be transmitted into IP packet through the use of IP (Internet Protocol); manages a data stream through the use of TCP (Transmission Control Protocol); performs a re-transmission of data by an ARQ (Automatic Retransmission Request) method; and configures the received data.
A TCP/IP protocol is for a wired network, which is not appropriate for the mobile WiMAX network for transmitting and receiving the data through wireless link. Especially, if a TCP ACK packet is wireless-transmitted by the present MAC layer, the data transmission performance is largely affected by the TCP ACK packet loss and transmission delay. That is, if the TCP ACK packet transmission is not optimized through the wireless link in the broadband wireless communication network, it is difficult to improve the data transmission performance.
The TCP ACK packet and data packet are transmitted separately. Thus, if the transmission frequency of TCP ACK packet is high and the transmission performance of TCP ACK packet is deteriorated, the throughput efficiency of payload data transmission is reduced. Also, if the loss of data to be transmitted may be high due to the characteristics of the wireless link, the lost data is re-transmitted by the ARQ (Automatic Retransmission Request) method. Generally, since the data to be re-transmitted is transmitted with priority, the delay of TCP ACK packet, that is, ACK compression may occur, whereby the upper layer (TCP/IP layer) transmission performance may be deteriorated.
In case of one-way traffic, the TCP packet transmitted in one direction may share the same physical path with the TCP packet transmitted in the opposite direction. In this case, the data and TCP ACK packet may share a buffer in the network.
The buffer sharing causes the ACK compression, to thereby delay the TCP ACK packet transmission. As a result, the delay of TCP ACK packet transmission affects the throughput of payload data transmission of a sender, to thereby deteriorate the end-to-end data transmission performance.
The data transmission performance in the traffic throughput under two-way connection may be dropped to 66.67% of that under one-way traffic in which TCP ACK packet and data (payload) packet flow is separated.
A mobile station (MS) and base station (BS) of the broadband wireless communication network can classify packets transmitted through a service flow (SF) by a connection identifier (hereinafter, referred to as ‘CID’). Also, a traffic queue applied in the MAC layer may be applied to uplink (UL) and downlink (DL) contained in the same CID or SF.
This may introduce the TCP ACK compression. In order to prevent the TCP ACK compression, an independent service flow for separation of uplink (UL) and downlink (DL) traffic has to be created. However, it is impractical to create the SF for every application/service between MS and BS.
For minimizing these problems, the following methods have been proposed: (1) method of splitting the connection and (2) link layer solution method.
First, (1) method of splitting the connection is a mechanism referred to as ‘Indirect-TCP’ for splitting one TCP connection into two separate connections. The drawback in (1) method of splitting the connection is that acknowledgements received by the sender do not always imply that the packets have successfully reached the intended destination, which violates the End-To-End concept of TCP protocol.
Second, (2) link layer solution method uses ARQ (Automatic Repeat reQuest) and FEC (Forward Error Correction) at the link layer so as to obtain local data reliability and make the wireless link appear to TCP as a reliable link. This method is advantageous in that any modification to the TCP layer is not required. However, if applying the two-way TCP connection, the packet delay may be increased in the MAC layer.